Non-destructive testing instruments are frequently designed to measure physical characteristics of an object in certain units which have been established according to certain standards. For example, electrical conductivity of a test part may be measured as a percentage of the conductivity of a certain standard specimen structure, permeability may be measured as a ratio to the permeability of air and length dimensions such as the thickness of a layer, for example, may be measured in conventional units of length.
Most non-destructive testing instruments have heretofore used meters with a movable pointer or the like. If in such instruments a signal is produced which does not have a linear relationship to the units being measured, the scale of the meter can be appropriately calibrated to compensate. There are certain problems in the use of meters, however, and as disclosed in John J. Flaherty U.S. Pat. No. 4,074,186, issued Feb. 14, 1978, such problems as are related to conductivity testing might be avoided by use of a digital read-out arrangement, provided that an analog voltage is developed as a linear function of conductivity.
The arrangements as disclosed in said patent constitute an important break-through in the measurement of conductivity but have limitations. Linearization circuits as disclosed are subject to inaccuracies or are quite complicated and expensive. A linearization circuit such as disclosed in FIG. 3 of the patent, which uses diodes, is satisfactory for many applications but where an extremely high degree of accuracy is desired, the characteristics of the diodes may preclude the attainment of the desired results. A circuit such as shown in FIG. 4 of the patent involving the use of an analog-to-digital converter, a read-only memory and a digital-to-analog converter can provide a very high degree of accuracy but has disadvantages in that the programming might have to be altered for different instruments or different probes and it would be impractical and expensive to individually program a read-only memory for each instrument.
Attempts have also been made to use a circuit according to FIG. 3 of the aforesaid Flaherty patent but using transistors in place of diodes but it was found that there are severe problems because the transistor base-to-emitter voltage current is not a linear function of collector current.